1. Field of the Invention
The present invention relates to a color cathode-ray tube, and particularly to an improvement of the shape of the slot apertures of a color cathode-ray tube that employs a shadow mask having slot apertures.
2. Description of the Related Art
A shadow mask-type color cathode-ray tube composed of a striped fluorescent screen and a slot-aperture shadow mask is a glass vacuum tube formed from panel and funnel 32, as well as neck tube 33, as shown in FIG. 1. Electron gun 34 is located inside neck tube 33, and three electron beams 5 corresponding to fluorescent stripes of the three colors red, green, and blue are emitted from this electron gun 34. Electron beams 5 are electromagnetically deflected by deflection yoke 36 arranged outside funnel 32 and transmitted through slot apertures of color-selection shadow mask 4. The beams strike the fluorescent stripes of three colors, red, green, and blue, formed on fluorescent screen 8 on the inner surface of panel 31, causing the fluorescent material in the fluorescent stripes to emit light and thus generate an image.
Typically, a combination of a striped fluorescent surface and a slot-aperture shadow mask has been employed in color cathode-ray tubes for television of the prior art, while a combination of a dotted fluorescent surface and a round-aperture shadow mask has been employed in color cathode-ray tubes for high-resolution display. As a standard, the shadow mask of a color cathode-ray tube for television has a horizontal pitch of approximately 0.8 mm and a vertical pitch of approximately 0.8 mm, these values varying somewhat depending on the screen size. In contrast, the pitch of a shadow mask for a high-resolution display has a reference value of approximately 0.27 mm.
Slot-aperture shadow masks are used in color cathode-ray tubes for television chiefly because a brighter image can be obtained than with other types of shadow masks, and because the beam landing margin in the vertical direction is essentially infinite when a slot aperture shadow mask is used in combination with a striped fluorescent surface, thereby simplifying the landing design.
In contrast, round-aperture shadow masks are used in high-resolution display tubes because, in fabricating shadow masks that enable high-resolution display, round-aperture shadow masks are easier to fabricate than slot aperture shadow masks, and because round-aperture shadow masks have a uniform mechanical strength, thereby simplifying the press forming of shadow masks.
However, it has been recently suggested that a striped fluorescent surface is more suitable as the fluorescent screen for high-resolution display tubes than a dot fluorescent surface (for example, SID, EURO Display 1996, p. 138, 11. 1-18). Not only because a striped fluorescent surface is superior for high-resolution, but also due to the greater efforts now being expended toward the development of slot-type high-resolution tubes having striped fluorescent screen as well as due to improvements in shadow mask fabrication technology and shadow mask press formation technology, high-resolution color cathode-ray tubes using slot-type shadow masks are now being fabricated. A standard slot-type shadow mask has a horizontal pitch of approximately 0.25 mm and a vertical pitch of approximately 0.25 mm.
This reduction in pitch, however, has given rise to problems not encountered in color cathode-ray tubes for television. Among these problems, the projection onto fluorescent screen 8 of electron beams bends into electron beam projection 7b, which is in an inwardly curving banana-like shape as shown in FIG. 2. This problem is encountered in the case that the electron beam is transmitted by the slot apertures of the shadow mask corresponding to the vicinities of the left and right ends of the display screen. FIG. 2 is an enlarged view of fluorescent screen 8 in the vicinity of the right end of a display screen. The projections of electron beams are actually straight continuously from the upper end to the lower end of fluorescent screen 8 in the order of green fluorescent stripe 9g and blue fluorescent stripe 9b, but electron beam projection 7b corresponding to slot apertures in the vicinities of both the right and left ends of the display screen is bent approximately 10 .mu.m in a banana-like shape.
If the electron beam projection is adjusted to achieve just landing of the central portion of the electron beam projection onto the fluorescent stripe, the upper and lower end areas of inwardly bending electron beam projection 7b will be shifted toward the center (inwardly) with respect to straight electron beam projection 7a as shown in FIG. 3. In addition, inner portions 11 at the upper end and lower end of electron beam projection 7b approach the adjacent stripe on the inward side and thus tend to strike the stripe of another color, resulting in color interference. Moreover, outward portions 12 (in the direction opposite the center when viewed in the horizontal direction of the screen) at the upper and lower portions of electron beam projection 7b deviate from the fluorescent stripe and therefore tend to give rise to areas of diminished light emission. This phenomenon was not recognized as a problem in color cathode-ray tubes for television.
To examine this phenomenon using actual numerical values, in a 17-inch high-resolution tube in which the shadow mask has a horizontal pitch of 0.25 mm, the width of the fluorescent stripes is approximately 42 .mu.m, the width of the graphite stripes is approximately 45 .mu.m, and the width of an electron beam projection is approximately 75 .mu.m. Even in cases in which the electron beam projection is straight and free of bending, a drop in luminance begins with a mislanding of 17 .mu.m with respect to a stripe of 42 .mu.m, and color interference (impingement on stripes of other colors) begins with a mislanding of 29 .mu.m. Accordingly, the banana-like bending of 10 .mu.m described above causes the landing margin to be reduce by 10 .mu.m, thereby causing color interference to occur, i.e., another color to be hit, with a mislanding of only 19 .mu.m.
A bend of approximately 10 .mu.m in the projection of an electron beam is consequently a considerably large value for luminance and landing margin in the case of a high-resolution tube, and the correction of this bending is extremely important for the production of a slot-type high-resolution tube.
Japanese Patent Laid-open No. 320738/89 relates to a color cathode-ray tube for television. The document points out the problem that, when the plate thickness of the shadow mask is increased in order to meet such requirements as larger size and wider deflection angle, the projection onto the fluorescent screen of the electron beams blocked by the inner wall of the slot transmission apertures becomes seed-shaped as a persimmon seed.
One method that has been proposed to solve this problem involves partially widening the slot apertures by retreating only the outer side-surface of the aperture in the outward direction of the screen, as shown in FIG. 4, so as to prevent the electron beam projection from being deformed by collision with the outer side-surface.
Japanese Patent Laid-open No. 6741/93 describes the blocking of the electron beam by the corner areas of the siedewalls of slot apertures as one cause for the deformation of a beam projection. To prevent this deformation and improve the beam form, the corners of slot transmission aperture 1 and front-side large aperture 2 are extended outward in the horizontal direction as shown in FIG. 5.
The problems disclosed in the above-described documents are similar to the previously described banana-shaped deformation in that the problems relate to the shape of the projection of an electron beam that is transmitted by a slot aperture of a shadow mask. However, the thickness of the shadow mask of the cited color cathode-ray tubes for television is relatively thick, from 0.15 to 0.18 mm or from 0.2 to 0.3 mm, and the persimmon-seed-shaped deformation of the electron beam projection differs from the above-described banana-shaped deformation in that it is generated when a portion of the electron beam strikes the sidewall surrounding the front-side large apertures of a thick shadow mask, as described below.
It is believed that the use of a thin shadow mask, measuring from 0.1 mm to 0.13 mm thick, in a high-resolution color cathode-ray tube greatly suppresses the persimmon-seed-shaped deformation. As will be explained hereinbelow, the banana-shaped deformation is caused because the minimum-width portions of a slot transmission aperture are not in the same plane, the positions of the upper and lower ends of the slot transmission aperture being at positions 30 .mu.m closer to the panel than the vertical center of the aperture, and because the incidence of the electron beam is more inclined in the horizontal direction at the two horizontal extremities of the shadow mask than at the middle thereof.
The banana-lice curvature of the electron beam projection in a high-resolution color cathode-ray tube occurs because the minimum-width portion of the slot transmission aperture, which actually determines the shape of beam projection, is not in the same plane, the minimum-width portion at the short sides of the slot transmission aperture being approximately 30 .mu.m closer to the inner surface of the panel than the minimum-width portion at the center of the long sides. As a result, of the electron beams diagonally incident on the slot transmission aperture, the electron beam that is transmitted by the portion of the slot aperture in the areas of the short sides (the portion closest to the panel) lands at a more inward point in the horizontal direction of the display screen than an electron beam that is transmitted by the central portion of the slot aperture, which is relatively far from the inner surface of the panel. An electron beam that is transmitted by the center of the long sides of a slot thus lands at a more outward point than a beam transmitted by the short sides. As a result, the projection of an electron beam takes on a banana-like shape in which the upper and lower ends are bent inwardly in the horizontal direction.
The banana-shaped bend of an electron beam projection in a high-resolution color cathode-ray tube is therefore not caused by the blocking of an electron beam by the wall around the slot aperture on the front side of the aperture, and as a result, both the cause and the problem of banana-shaped curvature of the projection of an electron beam transmitted by an aperture differ from the case of a color cathode-ray tube for television.
Lowering the positions of the minimum-width portions of the short sides of the slot apertures to the same height as those of the long sides can be considered as a countermeasure to this bending, but such a countermeasure would also entail an unacceptable loss in luminance due to widening of the bridge portions. Conversely, raising the position of the minimum width portions of the long sides to the same height as the position of the minimum-width portion of the short sides can also be considered, but this course would entail increased beam reflection from the inclined sidewall surfaces of the rear-side small aperture toward the fluorescent screen, thereby causing an undesirable reduction in contrast.
The problem to be solved by the present invention can therefore be summarized as follows:
The minimum-width portion of a slot transmission aperture of a shadow-mask slot aperture does not all lie in the same plane, the position of the minimum-width portion of the slot curving to approach the panel as the short-sides are approached from the center of the long sides. As a result, the projection onto the inner surface of the panel, of an electron beam that is transmitted by a slot aperture located in the vicinity of the right end on the horizontal axis of the active area of a shadow mask becomes electron beam projection 7b, in which the upper and lower ends bend toward the left with respect to the vertical center of the projection, as shown in FIG. 6. The electron beam projection therefore takes on a bent banana-like shape.
In the case of a 17-inch 90-degree deflection tube, the upper and lower ends of an electron beam projection shift by approximately 10 .mu.m to the left, and therefore tend to strike the adjacent stripe to the left. Landing margin therefore decreases and color interference occurs, thereby giving rise to the problem of the increased potential for a loss in color saturation. At the same time, the leftward shift of the electron beam projection in the vicinity of the upper and lower ends of the electron beam projection causes the beam to tend to miss on the right side of the target stripe, thereby giving rise to the problem of an increased potential for a drop in luminance and a loss in white uniformity. A decrease of 10 .mu.m in margin was negligible for a color cathode-ray tube for television in which the margin is great, but such a decrease is a problem that cannot be ignored in a high-resolution color cathode-ray tube in which the margin is small.